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When three identical bulbs of 60 watt, 200 volt rating are connected in series to a 200 volt supply, the power drawn by them will be

1. 20 watt
2. 60 watt
3. 180 watt
4. 10 watt

Correct answer: 20 watt

Solution

When connected in series, the total resistance of the bulbs is 3 times the resistance of one bulb. The resistance of one bulb is R = V²/P = (200)²/60 = 666.67 Ω. Total resistance = 3R = 2000 Ω. The current through the circuit is I = V/R = 200/2000 = 0.1 A. Power drawn = I²R = (0.1)² × 2000 = 20 W.

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